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I am trying to collect paginated results by trying to do the following logic in Scala and failed pathetically:
def python_version():
cursor
books, cursor = fetch_results()
while (cursor!=null) {
new_books = fetch_results(cursor)
books = books + new_books
}
return books
def fetch_results(cursor=None):
#do some fetchings...
return books, next_cursor
Here is an alternative solution using a recursive function, which avoids mutable values:
def fetchResults(c: Option[Cursor]=None): (List[Book], Option[Cursor]) = ...
def fetchAllResults(): List[Book] = {
#tailrec
def loop(cursor: Option[Cursor], res: List[Book]): List[Book] = {
val (books, newCursor) = fetchResults(cursor)
val newBooks = res ::: books
newCursor match {
case Some(_) =>
loop(newCursor, newBooks)
case None =>
newBooks
}
}
loop(None, Nil)
}
This is a fairly standard pattern for recursive functions in Scala where the actual recursion is done in an internal function. The result of the previous iteration is passed down to the next iteration and then returned from the function. This means that loop is a tail-recursive function and can be optimised by the compiler into a while loop. (The #tailrec annotation tells the compiler to warn if this is not actually tail-recursive)
Something like this, perhaps:
Iterator.iterate(fetch_results()) {
case (_, None) => (Nil, None)
case (books, cursor) => fetch_results(cursor)
}.map(_._1).takeWhile(_.nonEmpty).flatten.toList`
.iterate takes the first parameter to be the initial element of the iterator, and the second one is a function, that, given previous element, computes the next one.
So, this creates an iterator of tuples (Seq[Book], Cursor), starting with the initial return of fetch_results, and then keeps fetching more results, and accumulating them, until the nextCursor is None (I used None instead of null, because nulls are evil, and shouldn't really be used in a normal language, like scala, that provides enough facilities to avoid them).
Then .map(_._1) discards the cursors (don't need them any more), so we now have an iterator of pages, .takeWhile truncates the iterator at the first
page that is empty, then .flatten concatenates all inner Seqs together, and finally toList materializes all the elements, and returns the entire list of books.
I was given a question to compare two trees. Something like below:
case class Node(elem:String, child:List[Node])
In order to compare each elements of the trees, I have following functions:
def compare(n1:Node, n2:Node): Boolean {
if(n1.elem == n2.elem){
return compare(n1.child, n2.child)
}
}
def compare(c1:List[Node], c2:List[Node]): Boolean {
while (c1.notEmpty) {
//filter, map etc call function above to compare the element recursively
}
}
Basically algorithm is for each elements in n1, we are checking for a match in n2. I was told that this is very imperative way and not functional way. What would be a functional way to achieve this behaviour. In other words, how do we remove while loop when comparing the list of children?
Consider zipping both lists and using forall which holds true only if each and every predicate it processes evaluates to true; for instance like this,
def compare(c1: List[Node], c2: List[Node]): Boolean =
(c1.sorted zip c2.sorted).forall(c => compare(c._1,c._2))
Note that forall will halt the evaluations as it encounters the first false. Cases of unequal length lists of nodes may be tackled with zipAll by defining an EmptyNode class for padding length differences; also both lists empty may compare to true.
Update
Sorted lists of nodes for soundness following comment by #JohnB.
If I understood your question correctly, you want to compare every element of the first list with every element of the second list. The following code achieves this. It gets rid of the while loop via a tail-recursion.
import scala.annotation.tailrec
def cmp(a:Int, b:Int) = a > b
#tailrec
def compare(xs: List[Int], ys: List[Int]): Boolean = xs match {
case Nil => true
case head :: tail if ys.forall(x => cmp(head, x)) => compare(tail, ys)
case _ => false
}
I'm splitting an input of type Option[String] into an Option[Array[String]] as follows:
val input:Option[String] = Option("a=b,1000,what?")
val result: Option[Array[String]] = input map { _.split(",") }
I want to add a test whereby if any member of the array matches (eg, is an Long less than 0), the whole array is discarded and an empty Option returned.
Use filter to perform a test on the content of an Option.
Use exists to check whether any member of the collection fullfils a condition.
result.filter(! _.exists(s => test(s)))
or
result.filterNot(_.exists(s => test(s)))
Have you considered using find() on the collection ? If it returns a Some(x), then something has satisfied the condition.
list.find(_ < 0) match {
case Some(x) => None
case None => Some(list)
}
Of course you know that you can split and then filter as #ziggystar suggests, but if you have a really big Stringand an element at the beginning matches then it's pointless to finish splitting the string when you know it's going to be discarded.
In this case, if you're worried about time efficiency, you can use a Stream and re-implement the split operation, something like this:
def result(input:Option[String]):Option[Seq[String]] = {
def split(c: Char, chars:Stream[Char]):Stream[String] = {
val (head,tail) = chars span(_ != c)
head.mkString #:: (if(tail isEmpty) Stream.empty else split(c, tail tail))
}
input map {s => split(',', Stream(s:_*)) } filter (_.forall (s => !test(s)))
}
Note that the map/filter structure stays the same, but it is now short-circuiting due to the use of Stream.
If it's a really big string you probably have it as a Stream[Char] already which means you don't even have the memory overhead of hanging on the original String.
Trying to get a handle on pattern matching here-- coming from a C++/Java background it's very foreign to me.
The point of this branch is to check each member of a List d of tuples [format of (string,object). I want to define three cases.
1) If the counter in this function is larger than the size of the list (defined in another called acc), I want to return nothing (because there is no match)
2) If the key given in the input matches a tuple in the list, I want to return its value (or, whatever is stored in the tuple._2).
3) If there is no match, and there is still more list to iterate, increment and continue.
My code is below:
def get(key:String):Option[Any] = {
var counter: Int = 0
val flag: Boolean = false
x match {
case (counter > acc) => None
case ((d(counter)._1) == key) => d(counter)._2
case _ => counter += 1
}
My issue here is while the first case seems to compile correctly, the second throws an error:
:36: error: ')' expected but '.' found.
case ((d(counter)._1) == key) => d(counter)._2
The third as well:
scala> case _ => counter += 1
:1: error: illegal start of definition
But I assume it's because the second isn't correct. My first thought is that I'm not comparing tuples correctly, but I seem to be following the syntax for indexing into a tuple, so I'm stumped. Can anyone steer me in the right direction?
Hopefully a few things to clear up your confusion:
Matching in scala follows this general template:
x match {
case SomethingThatXIs if(SomeCondition) => SomeExpression
// rinse and repeat
// note that `if(SomeCondition)` is optional
}
It looks like you may have attempted to use the match/case expression as more of an if/else if/else kind of block, and as far as I can tell, the x doesn't really matter within said block. If that's the case, you might be fine with something like
case _ if (d(counter)._1 == key) => d(counter)._2
BUT
Some info on Lists in scala. You should always think of it like a LinkedList, where indexed lookup is an O(n) operation. Lists can be matched with a head :: tail format, and Nil is an empty list. For example:
val myList = List(1,2,3,4)
myList match {
case first :: theRest =>
// first is 1, theRest is List(2,3,4), which you can also express as
// 2 :: 3 :: 4 :: Nil
case Nil =>
// an empty list case
}
It looks like you're constructing a kind of ListMap, so I'll write up a more "functional"/"recursive" way of implementing your get method.
I'll assume that d is the backing list, of type List[(String, Any)]
def get(key: String): Option[Any] = {
def recurse(key: String, list: List[(String, Any)]): Option[Any] = list match {
case (k, value) :: _ if (key == k) => Some(value)
case _ :: theRest => recurse(key, theRest)
case Nil => None
}
recurse(key, d)
}
The three case statements can be explained as follows:
1) The first element in list is a tuple of (k, value). The rest of the list is matched to the _ because we don't care about it in this case. The condition asks if k is equal to the key we are looking for. In this case, we want to return the value from the tuple.
2) Since the first element didn't have the right key, we want to recurse. We don't care about the first element, but we want the rest of the list so that we can recurse with it.
3) case Nil means there's nothing in the list, which should mark "failure" and the end of the recursion. In this case we return None. Consider this the same as your counter > acc condition from your question.
Please don't hesitate to ask for further explanation; and if I've accidentally made a mistake (won't compile, etc), point it out and I will fix it.
I'm assuming that conditionally extracting part of a tuple from a list of tuples is the important part of your question, excuse me if I'm wrong.
First an initial point, in Scala we normally would use AnyRef instead of Object or, if worthwhile, we would use a type parameter which can increase reuse of the function or method and increase type safety.
The three cases you describe can be collapsed into two cases, the first case uses a guard (the if statement after the pattern match), the second case matches the entire non-empty list and searches for a match between each first tuple argument and the key, returning a Some[T] containing the second tuple argument of the matching tuple or None if no match occurred. The third case is not required as the find operation traverses (iterates over) the list.
The map operation after the find is used to extract the second tuple argument (map on an Option returns an Option), remove this operation and change the method's return type to Option[(String, T)] if you want the whole tuple returned.
def f[T](key: String, xs: List[(String, T)], initialCount: Int = 2): Option[T] = {
var counter = initialCount
xs match {
case l: List[(String, T)] if l.size < counter => None
case l: List[(String, T)] => l find {_._1 == key} map {_._2}
}
}
f("A", List(("A", 1), ("B", 2))) // Returns Some(1)
f("B", List(("A", 1), ("B", 2))) // Returns Some(2)
f("A", List(("A", 1))) // Returns None
f("C", List(("A", 1), ("B", 2))) // Returns None
f("C", Nil) // Returns None
First, why are you using a List for that reason? What you need is definitely a Map. Its get() returns None if key is not found and Some(value) if it is found in it.
Second, what is x in your example? Is it the list?
Third, you cannot write case (log) => .. where log is a logical condition, it is in the form of case _ if (log) => ... (as Rex Kerr already pinted out in his comment).
Fouth, you need a recursive function for this (simply increasing the counter will call this only on the second element).
So you'll need something like this (if still prefer sticking to List):
def get(l: List[Tuple2[String, String]], key: String): Option[String] = {
if (l.isEmpty) {
None
} else {
val act = l.head
act match {
case x if (act._1 == key) => Some(act._2)
case _ => get(l.tail, key)
}
}
}
I've obviously done a very poor job of explaining what I'm looking for in my original post so let's try this one more time. What I'm trying to accomplish is the ability to pass a sequence of items, extract one or more of the items, and then pass the REMAINDER of the sequence on to another extractor. Note that by sequence I mean sequence (not necessarily a List). My previous examples used list as the sequence and I gave some examples of extraction using cons (::), but I could just as well pass an Array as my sequence.
I thought I knew how pattern matching and extraction worked but I could be wrong so to avoid any more basic comments and links to how to do pattern matching sites here's my understanding:
If I want to return a single item from my extractor I would define an unapply method. This method takes whatever type I chose as input (the type could be a sequence...) and returns a single optional item (the return type could itself be a sequence). The return must be wrapped in Some if I want a match or None if I don't. Here is an example that takes a sequence as input and returns the same sequence wrapped in Some but only if it contains all Strings. I could very well just return the sequence wrapped in Some and not do anything else, but this seems to cause confusion for people. The key is if it is wrapped in Some then it will match and if it is None it will not. Just to be more clear, the match will also not happen unless the input also matches my unapply methods input type. Here is my example:
object Test {
// In my original post I just returned the Seq itself just to verify I
// had matched but many people commented they didn't understand what I
// was trying to do so I've made it a bit more complicated (e.g. match
// only if the sequence is a sequence of Strings). Hopefully I don't
// screw this up and introduce a bug :)
def unapply[A](xs: Seq[A]): Option[Seq[String]] =
if (xs forall { _.isInstanceOf[String] })
Some(xs.asInstanceOf[Seq[String]])
else
None
}
Using List as an example, I can now perform the following:
// This works
def test1(xs: List[_]) = xs match {
case (s: String) :: Test(rest) =>
println("s = " + s + ", rest = " + rest)
case _ =>
println("no match")
}
test1(List("foo", "bar", "baz")) // "s = foo, rest = List(bar, baz)"
My test1 function takes List as input and extracts the head and tail using cons via the constructor pattern (e.g. ::(s, rest)). It then uses type ascription (: String) to make sure the head (s) is a String. The tail contains List("bar", "baz"). This is a List which means it is also a Seq (sequence). It is then passed as input to my Test extractor which verifies that both "bar" and "baz" are strings and returns the List wrapped in Some. Since Some is returned it is considered a match (although in my original post where I inadvertently mixed up unapplySeq with unapply this didn't work as expected, but that aside...). This is NOT what I'm looking for. This was only an example to show that Test does in fact extract a Seq as input as expected.
Now, here's where I caused mass confusion last time when I inadvertently used unapplySeq instead of unapply in my write up. After much confusion trying to understand the comments that were posted I finally picked up on the mistake. Many thanks to Dan for pointing me in the right direction...
But just be avoid any more confusion, let me clarify my understanding of unapplySeq. Like unapply, unapplySeq takes in whatever argument I choose as input, but instead of returning a single element it returns a sequence of elements. Each item in this sequence can then be used for additional pattern matching. Again, to make a match happen the input type must match and my returned sequence must be wrapped in Some and not be None. When extracting over the sequence of items returned from unapplySeq, you can use _* to match any remaining items not yet matched.
Ok, so my extractor takes a sequence as input and returns a sequence (as a single item) in return. Since I only want to return a single item as a match I need to use unapply NOT unapplySeq. Even though in my case I'm returning a Seq, I don't want unapplySeq because I don't want to do more pattern matching on the items in the Seq. I just want to return the items as a Seq on its own to then be passed to the body of my case match. This sounds confusing, but to those that understand unapply vs unapplySeq I hope it isn't.
So here is what I WANT to do. I want to take something that returns a sequence (e.g. List or Array) and I want to extract a few items from this sequence and then extract the REMAINDER of the items (e.g. _*) as a sequence. Let's call it the remainder sequence. I want to then pass the remainder sequence as input to my extractor. My extractor will then return the remaining items as a single Seq if it matches my criteria. Just to be 100% clear. The List (or Array, etc) will have its unapplySeq extractor called to create the sequence of items. I will extract a one or more of these items and then pass what is left as a sequence to my Test extractor which will use unapply (NOT unapplySeq) to return the remainder. If you are confused by this, then please don't comment...
Here are my tests:
// Doesn't compile. Is there a syntax for this?
def test2(xs: Seq[_]) = xs match {
// Variations tried:
// Test(rest) # _* - doesn't compile (this one seems reasonable to me)
// Test(rest # _*) - doesn't compile (would compile if Test had
// unapplySeq, but in that case would bind List's
// second element to Test as a Seq and then bind
// rest to that Seq (if all strings) - not what I'm
// looking for...). I though that this might work
// since Scala knows Test has no unapplySeq only
// unapply so # _* can be tied to the List not Test
// rest # Test(_*) - doesn't compile (didn't expect to)
case List(s: String, Test(rest) # _*) =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
// This works, but messy
def test3(xs: List[_]) = xs match {
case List(s: String, rest # _*) if (
rest match { case Test(rest) => true; case _ => false }
) =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
I created test3 based on comments from Julian (thanks Julian..). Some have commented that test3 does what I want so they are confused what I'm looking for. Yes, it accomplishes what I want to accomplish, but I'm not satisfied with it. Daniel's example also works (thanks Daniel), but I'm also not satisfied with having to create another extractor to split things and then do embedded extractions. These solutions seem too much work in order to accomplish something that seems fairly straight forward to me. What I WANT is to make test2 work or know that it can't be done this way. Is the error given because the syntax is wrong? I know that rest # _* will return a Seq, that can be verified here:
def test4(xs: List[_]) = xs match {
case List(s: String, rest # _*) =>
println(rest.getClass) // scala.collection.immutable.$colon$colon
case _ =>
println("no match")
}
It returns cons (::) which is a List which is a Seq. So how can I pass the _* Seq on to my extractor and have is return bound to the variable rest?
Note that I've also tried passing varargs to my unapply constructor (e.g. unapply(xs: A*)...) but that won't match either.
So, I hope it is clear now when I say I want to extract the remainder of a sequence in pattern matching. I'm not sure how else I can word it.
Based on the great feedback from Daniel I'm hoping he is going to have an answer for me :)
I'd like to extract the first item and pass the remainder on to another extractor.
OK. Your test1 does that, exactly. first_item :: Extractor(the_rest). The weird behavior you're seeing comes from your Test extractor. As you already had the answer to your stated question, and as expected behavior from your Test strikes you as a problem with test1, it seems that what you really want is some help with extractors.
So, please read Extractor Objects, from docs.scala-lang.org, and Pattern Matching in Scala (pdf). Although that PDF has an example of unapplySeq, and suggests where you'd want to use it, here are some extra examples:
object Sorted {
def unapply(xs: Seq[Int]) =
if (xs == xs.sortWith(_ < _)) Some(xs) else None
}
object SortedSeq {
def unapplySeq(xs: Seq[Int]) =
if (xs == xs.sortWith(_ < _)) Some(xs) else None
}
Interactively:
scala> List(1,2,3,4) match { case Sorted(xs) => Some(xs); case _ => None }
res0: Option[Seq[Int]] = Some(List(1, 2, 3, 4))
scala> List(4,1,2,3) match { case Sorted(xs) => Some(xs); case _ => None }
res1: Option[Seq[Int]] = None
scala> List(4,1,2,3) match { case first :: Sorted(rest) => Some(first, rest); case _ => None }
res2: Option[(Int, Seq[Int])] = Some((4,List(1, 2, 3)))
scala> List(1,2,3,4) match { case SortedSeq(a,b,c,d) => (a,b,c,d) }
res3: (Int, Int, Int, Int) = (1,2,3,4)
scala> List(4,1,2,3) match { case _ :: SortedSeq(a, b, _*) => (a,b) }
res4: (Int, Int) = (1,2)
scala> List(1,2,3,4) match { case SortedSeq(a, rest # _*) => (a, rest) }
res5: (Int, Seq[Int]) = (1,List(2, 3, 4))
Or maybe -- I only have the faint suspicion of this, you haven't said as much -- you don't want extractor help, but actually you want a terse way to express something like
scala> List(1,2,3,4) match { case 1 :: xs if (xs match { case Sorted(_) => true; case _ => false }) => xs }
res6: List[Int] = List(2, 3, 4)
Erlang has a feature like this (although, without these crazy extractors):
example(L=[1|_]) -> examine(L).
, which pattern-matches the same argument twice - to L and also to [1|_]. In Erlang both sides of the = are full-fledged patterns and could be anything, and you can add a third or more patterns with more =. Scala seems to only support the L=[1|_] form, having a variable and then a full pattern.
scala> List(4,1,2,3) match { case xs # _ :: Sorted(_) => xs }
collection.immutable.::[Int] = List(4, 1, 2, 3)
Well, the easiest way is this:
case (s: String) :: Test(rest # _*) =>
If you need this to work on general Seq, you can just define an extractor to split head from tail:
object Split {
def unapply[T](xs: Seq[T]): Option[(T, Seq[T])] = if (xs.nonEmpty) Some(xs.head -> xs.tail) else None
}
And then use it like
case Split(s: String, Test(rest # _*)) =>
Also note that if you had defined unapply instead of unapplySeq, then # _* would not be required on the pattern matched by Test.
:: is an extractor. For how it works (from a random googling), see, for example, here.
def test1(xs: List[_]) = xs match {
case s :: rest =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
scala> test1(List("a", "b", "c"))
s = a rest = List(b, c)
I think this is what you wanted?
Messing around with this, it seems that the issue has something to do with unapplySeq.
object Test {
def unapply[A](xs: List[A]): Option[List[A]] = Some(xs)
}
def test1(xs: List[_]) = xs match {
case (s: String) :: Test(s2 :: rest) =>
println("s = " + s + " rest = " + rest)
case _ =>
println("no match")
}
test1(List("foo", "bar", "baz"))
produces the output:
s = foo rest = List(baz)
I'm havng trouble googling up docs on the difference between unapply and unapplySeq.